by Staff Writers
Cologne, Germany (SPX) May 25, 2017
Dipping a tube into a container filled with water will make the water rise in the tube. This phenomenon is called liquid capillarity. It is responsible for many natural and technical processes, for example the water absorption of trees, ink rising in a fountain pen, and sponges absorbing dishwater.
But what happens if the tube is dipped into a container filled not with water but with sand? The answer is - nothing. However, if the tube is shaken up and down, the sand will also begin to rise. Scientists have now discovered the mechanism behind this effect, the so-called granular capillary effect.
Dr Eric J. R. Parteli from the University of Cologne's Department of Geosciences, Professor Fengxian Fan from the University of Shanghai for Science and Technology, and Professor Thorsten Poschel from Friedrich-Alexander University Erlangen-Nurnberg have now published the results of their study 'Origin of Granular Capillarity Revealed by Particle-Based Simulations' in the Physical Review Letters.
Liquid capillarity results from the interplay of different molecular forces: the attraction between the liquid molecules keeps it together while the attraction between molecules and tube drives the liquid upward.
This explanation precludes the occurrence of capillarity for sand because sand grains are so much bigger than their constituent molecules that inter-molecular forces can be safely neglected compared to gravity and grain inertia.
However, surprisingly, granular capillarity has been observed in laboratory experiments in which the granular material was subjected to a tiny vertical vibration of a few grain diameters in amplitude and a frequency of just a few Hertz. The origin of this granular capillary effect was a long-standing mystery the international team of scientists succeed in unveiling.
They investigated the problem using a particle-based numerical simulation method called Discrete Element Method. In this method, the trajectory of every single grain is calculated by numerically solving Newton's equations of translational and rotational motion due to the forces that act on each grain.
By means of such a numerical experiment, it is thus possible to track the trajectory and velocity of all grains, including those grains that are deep within the granular bulk, which are difficult to assess in the laboratory.
The research team observed in their simulations that what makes the sand column ascend in the tube is a convective motion of the sand grains within the recipient that is inherent to granular materials under vertical vibrations.
This convective flux causes lateral mass transport within the vibrating granular packing, which leads to an upward pressure on the base of the granular column in the tube, which is why the column ascends.
The scientists found that how fast and far the column rises depends on the tube size. Remarkably, the simulations showed that the height of the granular meniscus (the capillary height that the granular column reaches after a long time) is proportional to the inverse of the tube size. This is exactly the same behaviour as for liquid capillarity, although the driving forces in the two systems are so much different.
The physicists showed in their study that the same capillary effect can be produced by shaking the tube instead of the container, which opens up promising applications in the handling and transportation sectors.
For example, particles could be pumped up from very large containers just by using granular capillarity. They are now studying the process in more depth to understand the effect of system and particle geometry.
Boston MA (SPX) May 24, 2017
A team of MIT researchers has designed a breathable workout suit with ventilating flaps that open and close in response to an athlete's body heat and sweat. These flaps, which range from thumbnail- to finger-sized, are lined with live microbial cells that shrink and expand in response to changes in humidity. The cells act as tiny sensors and actuators, driving the flaps to open when an athlete w ... read more
University of Cologne
Space Technology News - Applications and Research
|The content herein, unless otherwise known to be public domain, are Copyright 1995-2017 - Space Media Network. All websites are published in Australia and are solely subject to Australian law and governed by Fair Use principals for news reporting and research purposes. AFP, UPI and IANS news wire stories are copyright Agence France-Presse, United Press International and Indo-Asia News Service. ESA news reports are copyright European Space Agency. All NASA sourced material is public domain. Additional copyrights may apply in whole or part to other bona fide parties. All articles labeled "by Staff Writers" include reports supplied to Space Media Network by industry news wires, PR agencies, corporate press officers and the like. Such articles are individually curated and edited by Space Media Network staff on the basis of the report's information value to our industry and professional readership. Advertising does not imply endorsement, agreement or approval of any opinions, statements or information provided by Space Media Network on any Web page published or hosted by Space Media Network. Privacy Statement|